The phrase "live in sin" is typically used to refer to couples who are cohabiting without being married. However, there are several other synonyms that can be used to describe such a relationship. Some of these include "shacking up," "living together," "cohabitating," or "being in a domestic partnership." While these phrases may carry different connotations depending on cultural and religious contexts, they all refer to the same basic concept. It's important to remember that how one chooses to live their life and express their love is ultimately a personal decision, and that judgment based on societal or religious norms should be avoided.